Lots of people are moving from standalones to RetroArch, but considering how shitty and old the MAME driver is for RetroArch I'm really skeptical to consider them a direct threat.

It's more like the majority of emu people aren't interested in what MAME has to offer to begin with. Arcade and Apple II gamers are getting older and older. The console emulation on MAME is poor (that should be fixed) and the UI is kind of intimidating (I don't care if this is 'fixed'). RetroArch seems to attract people interested in consoles primarily, and MAME being there is just a bonus.